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After an error indication the throttle body was stuck open i have fitted a new throttle body the car was OK for about 500 miles then showed the boost pressure fault below and went into limp mode. Switch off and back on again and all was OK for a while.
A bit of internet research suggested this might be a vacuum leak so after tinkering some I replaced the gasket between the exhaust gas return pipe and the EGR as i re-used the old one to try and fix the error, but now it seems to run OK for a mile or so then shows the fault and goes into limp mode.

Questions:

Any ideas what could be causing the Boost pressure problem - could it be something to do with changing the throttle body or is that a coincidence?
I saw a suggestion of taking the vacuum line off the actuator on the turbo to test the N75 valve but if anything the car runs worse like that - even more gutless.
I notice i have an electronic vacuum actuator on the turbo - is there anyway to test these - do they fail often??

Thanks in advance.
Getting to the end of my tether with this one. Already had to cancel my holiday as the car isn't reliable

I have read here and elsewhere that the turbo VNT actuators on those engines are prone to fail, not had much to do with these engines personally, but you can check for split tubing etc and the actuator diaphragm with a hand operated vacuum pump or a bit tube and a good suck, AFAIK a new actuator needs to be set up with vcds. a bit more info at these links, sure you'll find more:

Thanks Rob. Just been playing with VCDS - i have bugger all boost pressure, logging required and actual. Required spikes from 1000 mbar to 2500 odd mbar, the actual stays around 900 and actually dips a little when the required spikes. Questions is why...
I've had all the vac pipes off and all look in good condition, and sucking on one end to pull a vacuum while blocking the other, all seem good - not very scientific...the one way valve from the vac pump seems ok too...
However if i suck on the pipe from the actuator though, its not open, but i can keep sucking which would imply its not sealed....and may be the cause...
How hard are these to set up? i have VCDS but have no clue how to set them up...
Might need to invest in a vacuum tester...

If you're sucking on the actuator and can keep drawing air then that suggests the actuator diaphragm is leaking, link in my previous post had replacement actuator costs. I found the same symptoms on my EGR actuator when that diaphragm was split. Sorry I can't tell you how to set these up, as I mentioned - not much experience on these engines.

need repair kit 03L198716A at £157.33, awkward to fit and MUST be accurately set up.

Dark was that day when Diesel conceived his grim engine that begot you, vile invention, more vicious, more criminal than the camera even, metallic monstrosity, bale and bane of our culture, chief woe of our Commonweal. How dare the Law prohibit hashish and heroin yet license your use, who inflate all weak inferior egos? Their addicts only do harm to their own lives: you poison the lungs of the innocent, your din dithers the peaceful, and on choked roads hundreds must daily die by chance-medley. Nimble technicians, surely you should hang your heads in shame. Your wit works mighty wonders, has landed men on the Moon, replaced brains by computers, and can smithy a "smart" bomb. It is a crying scandal that you cannot take the time or be bothered to build us, what sanity knows we need, an odorless and noiseless staid little electric brougham.

That was the pipe i was sucking on with not much resistance. Have ordered a vac tester from Amazon, will be here tomorrow, so can test it a bit more scientifically. I think i can just about see the actuator rod with a torch, and that didn't appear to move either.
So if the actuator fails - i suppose the vanes will be open, hence no boost pressure?

You have wasted your money ordering a vac pump, if you can suck air through that port the valve is FUBAR and therefore minimum boost.

Dark was that day when Diesel conceived his grim engine that begot you, vile invention, more vicious, more criminal than the camera even, metallic monstrosity, bale and bane of our culture, chief woe of our Commonweal. How dare the Law prohibit hashish and heroin yet license your use, who inflate all weak inferior egos? Their addicts only do harm to their own lives: you poison the lungs of the innocent, your din dithers the peaceful, and on choked roads hundreds must daily die by chance-medley. Nimble technicians, surely you should hang your heads in shame. Your wit works mighty wonders, has landed men on the Moon, replaced brains by computers, and can smithy a "smart" bomb. It is a crying scandal that you cannot take the time or be bothered to build us, what sanity knows we need, an odorless and noiseless staid little electric brougham.

It was a vac tester I ordered, sure it will get used again. Will have to ring the main dealer in the morning and see about getting it booked in. Not a job I fancy doing myself. Mant thanks for all the advice. Will post again with the ouToome.

An experienced VAG specialist would have the equipment and the experience. Oddly one of the most important tools is a curved 10-mm spanner.

Dark was that day when Diesel conceived his grim engine that begot you, vile invention, more vicious, more criminal than the camera even, metallic monstrosity, bale and bane of our culture, chief woe of our Commonweal. How dare the Law prohibit hashish and heroin yet license your use, who inflate all weak inferior egos? Their addicts only do harm to their own lives: you poison the lungs of the innocent, your din dithers the peaceful, and on choked roads hundreds must daily die by chance-medley. Nimble technicians, surely you should hang your heads in shame. Your wit works mighty wonders, has landed men on the Moon, replaced brains by computers, and can smithy a "smart" bomb. It is a crying scandal that you cannot take the time or be bothered to build us, what sanity knows we need, an odorless and noiseless staid little electric brougham.

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